Recovery USB stick failed with repeated 99s
I guess you fixed this by now but I'm surprised no one else helped.
The same thing happened to me and I too was puzzled why I got it work on one occasion and not another.
It turns out the USB creation process progress bar lies.
You have to wait until the "Completed" item on the left is checked off and not rely on the progress bar. Once the progress bar gets to 100%, you'll have to wait a good 2 to three minutes longer.
If you abort the creation process early and boot the USB stick you'll get the string "I" followed by endless "99"s.
I hope this helps others who stumbled as I did.